Method for acquiring synchronization in a communication system and device therefor

ABSTRACT

The present invention relates to a wireless communication system. More specifically, the present invention relates to a method for acquiring, by a first user equipment (UE) in a coverage of a first cell, synchronization in a wireless communication system, the method comprising: receiving a message for a sidelink resource configuration from a second cell through the second UE in a coverage of the second cell; if a channel quality of the second cell is less than a predetermined threshold, selecting the first cell as a synchronization reference cell; and transmitting a sidelink data to the second UE using the sidelink resource configuration based on the synchronization acquired from a synchronization signal of the first cell.

This application claims the benefit of the U.S. Provisional PatentApplication No. 62/367,079 filed on Jul. 26, 2016, which is herebyincorporated by reference as if fully set forth herein.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a wireless communication system and,more particularly, to a method for acquiring synchronization in acommunication system and a device therefor.

Discussion of the Related Art

As an example of a mobile communication system to which the presentinvention is applicable, a 3rd Generation Partnership Project Long TermEvolution (hereinafter, referred to as LTE) communication system isdescribed in brief.

FIG. 1 is a view schematically illustrating a network structure of anE-UMTS as an exemplary radio communication system. An Evolved UniversalMobile Telecommunications System (E-UMTS) is an advanced version of aconventional Universal Mobile Telecommunications System (UMTS) and basicstandardization thereof is currently underway in the 3GPP. E-UMTS may begenerally referred to as a Long Term Evolution (LTE) system. For detailsof the technical specifications of the UMTS and E-UMTS, reference can bemade to Release 7 and Release 8 of “3rd Generation Partnership Project;Technical Specification Group Radio Access Network”.

Referring to FIG. 1, the E-UMTS includes a User Equipment (UE), eNode Bs(eNBs), and an Access Gateway (AG) which is located at an end of thenetwork (E-UTRAN) and connected to an external network. The eNBs maysimultaneously transmit multiple data streams for a broadcast service, amulticast service, and/or a unicast service.

One or more cells may exist per eNB. The cell is set to operate in oneof bandwidths such as 1.25, 2.5, 5, 10, 15, and 20 MHz and provides adownlink (DL) or uplink (UL) transmission service to a plurality of UEsin the bandwidth. Different cells may be set to provide differentbandwidths. The eNB controls data transmission or reception to and froma plurality of UEs. The eNB transmits DL scheduling information of DLdata to a corresponding UE so as to inform the UE of a time/frequencydomain in which the DL data is supposed to be transmitted, coding, adata size, and hybrid automatic repeat and request (HARQ)-relatedinformation. In addition, the eNB transmits UL scheduling information ofUL data to a corresponding UE so as to inform the UE of a time/frequencydomain which may be used by the UE, coding, a data size, andHARQ-related information. An interface for transmitting user traffic orcontrol traffic may be used between eNBs. A core network (CN) mayinclude the AG and a network node or the like for user registration ofUEs. The AG manages the mobility of a UE on a tracking area (TA) basis.One TA includes a plurality of cells.

Although wireless communication technology has been developed to LTEbased on wideband code division multiple access (WCDMA), the demands andexpectations of users and service providers are on the rise. Inaddition, considering other radio access technologies under development,new technological evolution is required to secure high competitivenessin the future. Decrease in cost per bit, increase in serviceavailability, flexible use of frequency bands, a simplified structure,an open interface, appropriate power consumption of UEs, and the likeare required.

SUMMARY OF THE INVENTION

The object of the present invention can be achieved by providing amethod for acquiring, by a first user equipment (UE) in a coverage of afirst cell, synchronization in a wireless communication system, themethod comprising: receiving a message for a sidelink resourceconfiguration from a second cell through the second UE in a coverage ofthe second cell; if a channel quality of the second cell is less than apredetermined threshold, selecting the first cell as a synchronizationreference cell; and transmitting a sidelink data to the second UE usingthe sidelink resource configuration based on the synchronizationacquired from a synchronization signal of the first cell.

In another aspect of the present invention provided herein is first userequipment (UE) in a coverage of a first cell in a wireless communicationsystem, the first UE comprising: a transceiver; and a processorconnected with the transceiver, wherein the processor is configured to:control the transceiver to receive a message for a sidelink resourceconfiguration from a second cell through the second UE in a coverage ofthe second cell, if a channel quality of the second cell is less than apredetermined threshold, select the first cell as a synchronizationreference cell, and control the transceiver to transmit a sidelink datato the second UE using the sidelink resource configuration based on thesynchronization acquired from a synchronization signal of the firstcell.

Preferably, if the channel quality of the second cell is higher than orequal to the predetermined threshold, the second cell is selected as thesynchronization reference cell.

Preferably, the method further comprises measuring a channel quality ofthe second cell and one or more neighboring cells of the first UE.

Preferably, if the channel quality of the second cell is less than thepredetermined threshold, the first UE selects one of the one or moreneighboring cells as the synchronization reference cell based on thechannel quality.

Preferably, the method further comprises acquiring the synchronizationfrom the synchronization signal of the first cell.

Preferably, the predetermined threshold is received from the secondcell.

Preferably, the first UE is connected to the second cell through thesecond UE.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention.

FIG. 1 is a diagram showing a network structure of an Evolved UniversalMobile Telecommunications System (E-UMTS) as an example of a wirelesscommunication system;

FIG. 2A is a block diagram illustrating network structure of an evolveduniversal mobile telecommunication system (E-UMTS), and FIG. 2B is ablock diagram depicting architecture of a typical E-UTRAN and a typicalEPC;

FIG. 3 is a diagram showing a control plane and a user plane of a radiointerface protocol between a UE and an E-UTRAN based on a 3rd generationpartnership project (3GPP) radio access network standard;

FIG. 4 is a diagram of an example physical channel structure used in anE-UMTS system;

FIG. 5 is a diagram for a general overview of the LTE protocolarchitecture for the downlink;

FIG. 6 is an example of default data path for a normal communication;

FIGS. 7 and 8 are examples of data path scenarios for a proximitycommunication;

FIG. 9 is a conceptual diagram illustrating for a non-roaming referencearchitecture;

FIG. 10 is a conceptual diagram illustrating for a Layer 2 Structure forSidelink;

FIG. 11a is a conceptual diagram illustrating for User-Plane protocolstack for ProSe Direct Communication, and FIG. 11b is Control-Planeprotocol stack for ProSe Direct Communication;

FIG. 12 is a conceptual diagram illustrating for a PC5 interface forProSe Direct Discovery;

FIG. 13 is an exemplary diagram for explaining the D2D environment;

FIG. 14 is a flow chart according to an embodiment of the presentinvention;

FIG. 15 is a flowchart for explaining an embodiment of the presentinvention;

FIG. 16 is a block diagram of a communication apparatus according to anembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Universal mobile telecommunications system (UMTS) is a 3rd Generation(3G) asynchronous mobile communication system operating in wideband codedivision multiple access (WCDMA) based on European systems, globalsystem for mobile communications (GSM) and general packet radio services(GPRS). The long-term evolution (LTE) of UMTS is under discussion by the3rd generation partnership project (3GPP) that standardized UMTS.

The 3GPP LTE is a technology for enabling high-speed packetcommunications. Many schemes have been proposed for the LTE objectiveincluding those that aim to reduce user and provider costs, improveservice quality, and expand and improve coverage and system capacity.The 3G LTE requires reduced cost per bit, increased serviceavailability, flexible use of a frequency band, a simple structure, anopen interface, and adequate power consumption of a terminal as anupper-level requirement.

Hereinafter, structures, operations, and other features of the presentinvention will be readily understood from the embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings. Embodiments described later are examples in which technicalfeatures of the present invention are applied to a 3GPP system.

Although the embodiments of the present invention are described using along term evolution (LTE) system and a LTE-advanced (LTE-A) system inthe present specification, they are purely exemplary. Therefore, theembodiments of the present invention are applicable to any othercommunication system corresponding to the above definition. In addition,although the embodiments of the present invention are described based ona frequency division duplex (FDD) scheme in the present specification,the embodiments of the present invention may be easily modified andapplied to a half-duplex FDD (H-FDD) scheme or a time division duplex(TDD) scheme.

FIG. 2A is a block diagram illustrating network structure of an evolveduniversal mobile telecommunication system (E-UMTS). The E-UMTS may bealso referred to as an LTE system. The communication network is widelydeployed to provide a variety of communication services such as voice(VoIP) through IMS and packet data.

As illustrated in FIG. 2A, the E-UMTS network includes an evolved UMTSterrestrial radio access network (E-UTRAN), an Evolved Packet Core (EPC)and one or more user equipment. The E-UTRAN may include one or moreevolved NodeB (eNodeB) 20, and a plurality of user equipment (UE) 10 maybe located in one cell. One or more E-UTRAN mobility management entity(MME)/system architecture evolution (SAE) gateways 30 may be positionedat the end of the network and connected to an external network.

As used herein, “downlink” refers to communication from eNodeB 20 to UE10, and “uplink” refers to communication from the UE to an eNodeB. UE 10refers to communication equipment carried by a user and may be alsoreferred to as a mobile station (MS), a user terminal (UT), a subscriberstation (SS) or a wireless device.

FIG. 2B is a block diagram depicting architecture of a typical E-UTRANand a typical EPC.

As illustrated in FIG. 2B, an eNodeB 20 provides end points of a userplane and a control plane to the UE 10. MME/SAE gateway 30 provides anend point of a session and mobility management function for UE 10. TheeNodeB and MME/SAE gateway may be connected via an S1 interface.

The eNodeB 20 is generally a fixed station that communicates with a UE10, and may also be referred to as a base station (BS) or an accesspoint. One eNodeB 20 may be deployed per cell. An interface fortransmitting user traffic or control traffic may be used between eNodeBs20.

The MME provides various functions including NAS signaling to eNodeBs20, NAS signaling security, AS Security control, Inter CN node signalingfor mobility between 3GPP access networks, Idle mode UE Reachability(including control and execution of paging retransmission), TrackingArea list management (for UE in idle and active mode), PDN GW andServing GW selection, MME selection for handovers with MME change, SGSNselection for handovers to 2G or 3G 3GPP access networks, Roaming,Authentication, Bearer management functions including dedicated bearerestablishment, Support for PWS (which includes ETWS and CMAS) messagetransmission. The SAE gateway host provides assorted functions includingPer-user based packet filtering (by e.g. deep packet inspection), LawfulInterception, UE IP address allocation, Transport level packet markingin the downlink, UL and DL service level charging, gating and rateenforcement, DL rate enforcement based on APN-AMBR. For clarity MME/SAEgateway 30 will be referred to herein simply as a “gateway,” but it isunderstood that this entity includes both an MME and an SAE gateway.

A plurality of nodes may be connected between eNodeB 20 and gateway 30via the S1 interface. The eNodeBs 20 may be connected to each other viaan X2 interface and neighboring eNodeBs may have a meshed networkstructure that has the X2 interface.

As illustrated, eNodeB 20 may perform functions of selection for gateway30, routing toward the gateway during a Radio Resource Control (RRC)activation, scheduling and transmitting of paging messages, schedulingand transmitting of Broadcast Channel (BCCH) information, dynamicallocation of resources to UEs 10 in both uplink and downlink,configuration and provisioning of eNodeB measurements, radio bearercontrol, radio admission control (RAC), and connection mobility controlin LTE_ACTIVE state. In the EPC, and as noted above, gateway 30 mayperform functions of paging origination, LTE-IDLE state management,ciphering of the user plane, System Architecture Evolution (SAE) bearercontrol, and ciphering and integrity protection of Non-Access Stratum(NAS) signaling.

The EPC includes a mobility management entity (MME), a serving-gateway(S-GW), and a packet data network-gateway (PDN-GW). The MME hasinformation about connections and capabilities of UEs, mainly for use inmanaging the mobility of the UEs. The S-GW is a gateway having theE-UTRAN as an end point, and the PDN-GW is a gateway having a packetdata network (PDN) as an end point.

FIG. 3 is a diagram showing a control plane and a user plane of a radiointerface protocol between a UE and an E-UTRAN based on a 3GPP radioaccess network standard. The control plane refers to a path used fortransmitting control messages used for managing a call between the UEand the E-UTRAN. The user plane refers to a path used for transmittingdata generated in an application layer, e.g., voice data or Internetpacket data.

A physical (PHY) layer of a first layer provides an information transferservice to a higher layer using a physical channel. The PHY layer isconnected to a medium access control (MAC) layer located on the higherlayer via a transport channel. Data is transported between the MAC layerand the PHY layer via the transport channel. Data is transported betweena physical layer of a transmitting side and a physical layer of areceiving side via physical channels. The physical channels use time andfrequency as radio resources. In detail, the physical channel ismodulated using an orthogonal frequency division multiple access (OFDMA)scheme in downlink and is modulated using a single carrier frequencydivision multiple access (SC-FDMA) scheme in uplink.

The MAC layer of a second layer provides a service to a radio linkcontrol (RLC) layer of a higher layer via a logical channel. The RLClayer of the second layer supports reliable data transmission. Afunction of the RLC layer may be implemented by a functional block ofthe MAC layer. A packet data convergence protocol (PDCP) layer of thesecond layer performs a header compression function to reduceunnecessary control information for efficient transmission of anInternet protocol (IP) packet such as an IP version 4 (IPv4) packet oran IP version 6 (IPv6) packet in a radio interface having a relativelysmall bandwidth.

A radio resource control (RRC) layer located at the bottom of a thirdlayer is defined only in the control plane. The RRC layer controlslogical channels, transport channels, and physical channels in relationto configuration, re-configuration, and release of radio bearers (RBs).An RB refers to a service that the second layer provides for datatransmission between the UE and the E-UTRAN. To this end, the RRC layerof the UE and the RRC layer of the E-UTRAN exchange RRC messages witheach other.

One cell of the eNB is set to operate in one of bandwidths such as 1.25,2.5, 5, 10, 15, and 20 MHz and provides a downlink or uplinktransmission service to a plurality of UEs in the bandwidth. Differentcells may be set to provide different bandwidths.

Downlink transport channels for transmission of data from the E-UTRAN tothe UE include a broadcast channel (BCH) for transmission of systeminformation, a paging channel (PCH) for transmission of paging messages,and a downlink shared channel (SCH) for transmission of user traffic orcontrol messages. Traffic or control messages of a downlink multicast orbroadcast service may be transmitted through the downlink SCH and mayalso be transmitted through a separate downlink multicast channel (MCH).

Uplink transport channels for transmission of data from the UE to theE-UTRAN include a random access channel (RACH) for transmission ofinitial control messages and an uplink SCH for transmission of usertraffic or control messages. Logical channels that are defined above thetransport channels and mapped to the transport channels include abroadcast control channel (BCCH), a paging control channel (PCCH), acommon control channel (CCCH), a multicast control channel (MCCH), and amulticast traffic channel (MTCH).

FIG. 4 is a view showing an example of a physical channel structure usedin an E-UMTS system. A physical channel includes several subframes on atime axis and several subcarriers on a frequency axis. Here, onesubframe includes a plurality of symbols on the time axis. One subframeincludes a plurality of resource blocks and one resource block includesa plurality of symbols and a plurality of subcarriers. In addition, eachsubframe may use certain subcarriers of certain symbols (e.g., a firstsymbol) of a subframe for a physical downlink control channel (PDCCH),that is, an L1/L2 control channel. In FIG. 4, an L1/L2 controlinformation transmission area (PDCCH) and a data area (PDSCH) are shown.In one embodiment, a radio frame of 10 ms is used and one radio frameincludes 10 subframes. In addition, one subframe includes twoconsecutive slots. The length of one slot may be 0.5 ms. In addition,one subframe includes a plurality of OFDM symbols and a portion (e.g., afirst symbol) of the plurality of OFDM symbols may be used fortransmitting the L1/L2 control information. A transmission time interval(TTI) which is a unit time for transmitting data is 1 ms.

A base station and a UE mostly transmit/receive data via a PDSCH, whichis a physical channel, using a DL-SCH which is a transmission channel,except a certain control signal or certain service data. Informationindicating to which UE (one or a plurality of UEs) PDSCH data istransmitted and how the UE receive and decode PDSCH data is transmittedin a state of being included in the PDCCH.

For example, in one embodiment, a certain PDCCH is CRC-masked with aradio network temporary identity (RNTI) “A” and information about datais transmitted using a radio resource “B” (e.g., a frequency location)and transmission format information “C” (e.g., a transmission blocksize, modulation, coding information or the like) via a certainsubframe. Then, one or more UEs located in a cell monitor the PDCCHusing its RNTI information. And, a specific UE with RNTI “A” reads thePDCCH and then receive the PDSCH indicated by B and C in the PDCCHinformation.

FIG. 5 is a diagram for a general overview of the LTE protocolarchitecture for the downlink.

A general overview of the LTE protocol architecture for the downlink isillustrated in FIG. 5. Furthermore, the LTE protocol structure relatedto uplink transmissions is similar to the downlink structure in FIG. 5,although there are differences with respect to transport formatselection and multi-antenna transmission.

Data to be transmitted in the downlink enters in the form of IP packetson one of the SAE bearers (501). Prior to transmission over the radiointerface, incoming IP packets are passed through multiple protocolentities, summarized below and described in more detail in the followingsections:

-   -   Packet Data Convergence Protocol (PDCP, 503) performs IP header        compression to reduce the number of bits necessary to transmit        over the radio interface. The header-compression mechanism is        based on ROHC, a standardized header-compression algorithm used        in WCDMA as well as several other mobile-communication        standards. PDCP (503) is also responsible for ciphering and        integrity protection of the transmitted data. At the receiver        side, the PDCP protocol performs the corresponding deciphering        and decompression operations. There is one PDCP entity per radio        bearer configured for a mobile terminal.    -   Radio Link Control (RLC, 505) is responsible for        segmentation/concatenation, retransmission handling, and        in-sequence delivery to higher layers. Unlike WCDMA, the RLC        protocol is located in the eNodeB since there is only a single        type of node in the LTE radio-access-network architecture. The        RLC (505) offers services to the PDCP (503) in the form of radio        bearers. There is one RLC entity per radio bearer configured for        a terminal.

There is one RLC entity per logical channel configured for a terminal,where each RLC entity is responsible for: i) segmentation,concatenation, and reassembly of RLC SDUs; ii) RLC retransmission; andiii) in-sequence delivery and duplicate detection for the correspondinglogical channel.

Other noteworthy features of the RLC are: (1) the handling of varyingPDU sizes; and (2) the possibility for close interaction between thehybrid-ARQ and RLC protocols. Finally, the fact that there is one RLCentity per logical channel and one hybrid-ARQ entity per componentcarrier implies that one RLC entity may interact with multiplehybrid-ARQ entities in the case of carrier aggregation.

The purpose of the segmentation and concatenation mechanism is togenerate RLC PDUs of appropriate size from the incoming RLC SDUs. Onepossibility would be to define a fixed PDU size, a size that wouldresult in a compromise. If the size were too large, it would not bepossible to support the lowest data rates. Also, excessive padding wouldbe required in some scenarios. A single small PDU size, however, wouldresult in a high overhead from the header included with each PDU. Toavoid these drawbacks, which is especially important given the verylarge dynamic range of data rates supported by LTE, the RLC PDU sizevaries dynamically.

In process of segmentation and concatenation of RLC SDUs into RLC PDUs,a header includes, among other fields, a sequence number, which is usedby the reordering and retransmission mechanisms. The reassembly functionat the receiver side performs the reverse operation to reassemble theSDUs from the received PDUs.

-   -   Medium Access Control (MAC, 507) handles hybrid-ARQ        retransmissions and uplink and downlink scheduling. The        scheduling functionality is located in the eNodeB, which has one        MAC entity per cell, for both uplink and downlink. The        hybrid-ARQ protocol part is present in both the transmitting and        receiving end of the MAC protocol. The MAC (507) offers services        to the RLC (505) in the form of logical channels (509).    -   Physical Layer (PHY, 511), handles coding/decoding,        modulation/demodulation, multi-antenna mapping, and other        typical physical layer functions. The physical layer (511)        offers services to the MAC layer (507) in the form of transport        channels (513).

The Logical Channel Prioritization procedure is applied when a newtransmission is performed.

RRC controls the scheduling of uplink data by signalling for eachlogical channel: priority where an increasing priority value indicates alower priority level, prioritisedBitRate which sets the Prioritized BitRate (PBR), bucketSizeDuration which sets the Bucket Size Duration(BSD).

The MAC entity shall maintain a variable Bj for each logical channel j.Bj shall be initialized to zero when the related logical channel isestablished, and incremented by the product PBR×TTI duration for eachTTI, where PBR is Prioritized Bit Rate of logical channel j. However,the value of Bj can never exceed the bucket size and if the value of Bjis larger than the bucket size of logical channel j, it shall be set tothe bucket size. The bucket size of a logical channel is equal toPBR×BSD, where PBR and BSD are configured by upper layers.

The MAC entity shall perform the following Logical ChannelPrioritization procedure when a new transmission is performed. The MACentity shall allocate resources to the logical channels in the followingsteps.

Step 1: All the logical channels with Bj>0 are allocated resources in adecreasing priority order. If the PBR of a logical channel is set to“infinity”, the MAC entity shall allocate resources for all the datathat is available for transmission on the logical channel before meetingthe PBR of the lower priority logical channels.

Step 2: the MAC entity shall decrement Bj by the total size of MAC SDUsserved to logical channel j in Step 1.

Step 3: if any resources remain, all the logical channels are served ina strict decreasing priority order (regardless of the value of Bj) untileither the data for that logical channel or the UL grant is exhausted,whichever comes first. Logical channels configured with equal priorityshould be served equally.

The UE shall also follow the rules below during the schedulingprocedures above.

-   -   The UE should not segment an RLC SDU (or partially transmitted        SDU or retransmitted RLC PDU) if the whole SDU (or partially        transmitted SDU or retransmitted RLC PDU) fits into the        remaining resources of the associated MAC entity.    -   If the UE segments an RLC SDU from the logical channel, it shall        maximize the size of the segment to fill the grant of the        associated MAC entity as much as possible.    -   The UE should maximize the transmission of data.    -   if the MAC entity is given an UL grant size that is equal to or        larger than 4 bytes while having data available for        transmission, the MAC entity shall not transmit only padding BSR        and/or padding (unless the UL grant size is less than 7 bytes        and an AMD PDU segment needs to be transmitted).

The MAC entity shall not transmit data for a logical channelcorresponding to a radio bearer that is suspended.

For the Logical Channel Prioritization procedure, the MAC entity shalltake into account the following relative priority in decreasing order.

-   -   MAC control element for C-RNTI or data from UL-CCCH;    -   MAC control element for BSR, with exception of BSR included for        padding;    -   MAC control element for PHR, Extended PHR, or Dual Connectivity        PHR;    -   MAC control element for Sidelink BSR, with exception of Sidelink        BSR included for padding;    -   data from any Logical Channel, except data from UL-CCCH;    -   MAC control element for BSR included for padding;    -   MAC control element for Sidelink BSR included for padding.

When the MAC entity is requested to transmit multiple MAC PDUs in oneTTI, steps 1 to 3 and the associated rules may be applied either to eachgrant independently or to the sum of the capacities of the grants. Alsothe order in which the grants are processed is left up to UEimplementation. It is up to the UE implementation to decide in which MACPDU a MAC control element is included when MAC entity is requested totransmit multiple MAC PDUs in one TTI. When the UE is requested togenerate MAC PDU(s) in two MAC entities in one TTI, it is up to UEimplementation in which order the grants are processed.

FIG. 6 is an example of default data path for communication between twoUEs. With reference to FIG. 6, even when two UEs (e.g., UE1, UE2) inclose proximity communicate with each other, their data path (userplane) goes via the operator network. Thus a typical data path for thecommunication involves eNB(s) and/or Gateway(s) (GW(s)) (e.g., SGW/PGW).

FIGS. 7 and 8 are examples of data path scenarios for a proximitycommunication. If wireless devices (e.g., UE1, UE2) are in proximity ofeach other, they may be able to use a direct mode data path (FIG. 7) ora locally routed data path (FIG. 8). In the direct mode data path,wireless devices are connected directly each other (after appropriateprocedure(s), such as authentication), without eNB and SGW/PGW. In thelocally routed data path, wireless devices are connected each otherthrough eNB only.

FIG. 9 is a conceptual diagram illustrating for a non-roaming referencearchitecture.

PC1 to PC5 represent interfaces. PC1 is a reference point between aProSe application in a UE and a ProSe App server. It is used to defineapplication level signaling requirements. PC2 is a reference pointbetween the ProSe App Server and the ProSe Function. It is used todefine the interaction between ProSe App Server and ProSe functionalityprovided by the 3GPP EPS via ProSe Function. One example may be forapplication data updates for a ProSe database in the ProSe Function.Another example may be data for use by ProSe App Server in interworkingbetween 3GPP functionality and application data, e.g. name translation.PC3 is a reference point between the UE and ProSe Function. It is usedto define the interaction between UE and ProSe Function. An example maybe to use for configuration for ProSe discovery and communication. PC4is a reference point between the EPC and ProSe Function. It is used todefine the interaction between EPC and ProSe Function. Possible usecases may be when setting up a one-to-one communication path between UEsor when validating ProSe services (authorization) for session managementor mobility management in real time.

PC5 is a reference point between UE to UE used for control and userplane for discovery and communication, for relay and one-to-onecommunication (between UEs directly and between UEs over LTE-Uu).Lastly, PC6 is a reference point may be used for functions such as ProSeDiscovery between users subscribed to different PLMNs.

EPC (Evolved Packet Core) includes entities such as MME, S-GW, P-GW,PCRF, HSS etc. The EPC here represents the E-UTRAN Core Networkarchitecture. Interfaces inside the EPC may also be impacted albeit theyare not explicitly shown in FIG. 9.

Application servers, which are users of the ProSe capability forbuilding the application functionality, e.g. in the Public Safety casesthey can be specific agencies (PSAP) or in the commercial cases socialmedia. These applications are defined outside the 3GPP architecture butthere may be reference points towards 3GPP entities. The Applicationserver can communicate towards an application in the UE.

Applications in the UE use the ProSe capability for building theapplication functionality. Example may be for communication betweenmembers of Public Safety groups or for social media application thatrequests to find buddies in proximity. The ProSe Function in the network(as part of EPS) defined by 3GPP has a reference point towards the ProSeApp Server, towards the EPC and the UE.

The functionality may include but not restricted to e.g.:

-   -   Interworking via a reference point towards the 3rd party        Applications    -   Authorization and configuration of the UE for discovery and        Direct communication    -   Enable the functionality of the EPC level ProSe discovery    -   ProSe related new subscriber data and/handling of data storage;        also handling of ProSe identities;    -   Security related functionality    -   Provide Control towards the EPC for policy related functionality    -   Provide functionality for charging (via or outside of EPC, e.g.        offline charging)

Especially, the following identities are used for ProSe DirectCommunication:

-   -   Source Layer-2 ID identifies a sender of a D2D packet at PC5        interface. The Source Layer-2 ID is used for identification of        the receiver RLC UM entity;    -   Destination Layer-2 ID identifies a target of the D2D packet at        PC5 interface. The Destination Layer-2 ID is used for filtering        of packets at the MAC layer. The Destination Layer-2 ID may be a        broadcast, groupcast or unicast identifier; and    -   SA L1 ID identifier in Scheduling Assignment (SA) at PC5        interface. SA L1 ID is used for filtering of packets at the        physical layer. The SA L1 ID may be a broadcast, groupcast or        unicast identifier.

No Access Stratum signaling is required for group formation and toconfigure Source Layer-2 ID and Destination Layer-2 ID in the UE. Thisinformation is provided by higher layers.

In case of groupcast and unicast, the MAC layer will convert the higherlayer ProSe ID (i.e. ProSe Layer-2 Group ID and ProSe UE ID) identifyingthe target (Group, UE) into two bit strings of which one can beforwarded to the physical layer and used as SA L1 ID whereas the otheris used as Destination Layer-2 ID. For broadcast, L2 indicates to L1that it is a broadcast transmission using a pre-defined SA L1 ID in thesame format as for group- and unicast.

FIG. 10 is a conceptual diagram illustrating for a Layer 2 structure forSidelink. The Sidelink is UE to UE interface for ProSe directcommunication and ProSe Direct Discovery. Correspond to the PC5interface. The Sidelink comprises ProSe Direct Discovery and ProSeDirect Communication between UEs. The Sidelink uses uplink resources andphysical channel structure similar to uplink transmissions. However,some changes, noted below, are made to the physical channels. E-UTRAdefines two MAC entities; one in the UE and one in the E-UTRAN. TheseMAC entities handle the following transport channels additionally, i)sidelink broadcast channel (SL-BCH), ii) sidelink discovery channel(SL-DCH) and iii) sidelink shared channel (SL-SCH).

-   -   Basic transmission scheme: the Sidelink transmission uses the        same basic transmission scheme as the UL transmission scheme.        However, sidelink is limited to single cluster transmissions for        all the sidelink physical channels. Further, sidelink uses a 1        symbol gap at the end of each sidelink sub-frame.    -   Physical-layer processing: the Sidelink physical layer        processing of transport channels differs from UL transmission in        the following steps:

i) Scrambling: for PSDCH and PSCCH, the scrambling is not UE-specific;

ii) Modulation: 64 QAM is not supported for Sidelink.

-   -   Physical Sidelink control channel: PSCCH is mapped to the        Sidelink control resources. PSCCH indicates resource and other        transmission parameters used by a UE for PSSCH.    -   Sidelink reference signals: for PSDCH, PSCCH and PSSCH        demodulation, reference signals similar to uplink demodulation        reference signals are transmitted in the 4th symbol of the slot        in normal CP and in the 3rd symbol of the slot in extended        cyclic prefix. The Sidelink demodulation reference signals        sequence length equals the size (number of sub-carriers) of the        assigned resource. For PSDCH and PSCCH, reference signals are        created based on a fixed base sequence, cyclic shift and        orthogonal cover code.    -   Physical channel procedure: for in-coverage operation, the power        spectral density of the sidelink transmissions can be influenced        by the eNB.

FIG. 11a is a conceptual diagram illustrating for User-Plane protocolstack for ProSe Direct Communication, and FIG. 11b is Control-Planeprotocol stack for ProSe Direct Communication.

FIG. 11a shows the protocol stack for the user plane, where PDCP, RLCand MAC sublayers (terminate at the other UE) perform the functionslisted for the user plane (e.g. header compression, HARQretransmissions). The PC5 interface consists of PDCP, RLC, MAC and PHYas shown in FIG. 11 a.

User plane details of ProSe Direct Communication: i) MAC sub headercontains LCIDs (to differentiate multiple logical channels), ii) The MACheader comprises a Source Layer-2 ID and a Destination Layer-2 ID, iii)At MAC Multiplexing/demultiplexing, priority handling and padding areuseful for ProSe Direct communication, iv) RLC UM is used for ProSeDirect communication, v) Segmentation and reassembly of RLC SDUs areperformed, vi) A receiving UE needs to maintain at least one RLC UMentity per transmitting peer UE, vii) An RLC UM receiver entity does notneed to be configured prior to reception of the first RLC UM data unit,and viii) U-Mode is used for header compression in PDCP for ProSe DirectCommunication.

FIG. 11b shows the protocol stack for the control plane, where RRC, RLC,MAC, and PHY sublayers (terminate at the other UE) perform the functionslisted for the control plane. A D2D UE does not establish and maintain alogical connection to receiving D2D UEs prior to a D2D communication.

FIG. 12 is a conceptual diagram illustrating for a PC5 interface forProSe Direct Discovery.

ProSe Direct Discovery is defined as the procedure used by theProSe-enabled UE to discover other ProSe-enabled UE(s) in its proximityusing E-UTRA direct radio signals via PC5.

Radio Protocol Stack (AS) for ProSe Direct Discovery is shown in FIG.12.

The AS layer performs the following functions:

-   -   Interfaces with upper layer (ProSe Protocol): The MAC layer        receives the discovery information from the upper layer (ProSe        Protocol). The IP layer is not used for transmitting the        discovery information.    -   Scheduling: The MAC layer determines the radio resource to be        used for announcing the discovery information received from        upper layer.    -   Discovery PDU generation: The MAC layer builds the MAC PDU        carrying the discovery information and sends the MAC PDU to the        physical layer for transmission in the determined radio        resource. No MAC header is added.

There are two types of resource allocation for discovery informationannouncement.

-   -   Type 1: A resource allocation procedure where resources for        announcing of discovery information are allocated on a non UE        specific basis, further characterized by: i) The eNB provides        the UE(s) with the resource pool configuration used for        announcing of discovery information. The configuration may be        signalled in SIB, ii) The UE autonomously selects radio        resource(s) from the indicated resource pool and announce        discovery information, iii) The UE can announce discovery        information on a randomly selected discovery resource during        each discovery period.    -   Type 2: A resource allocation procedure where resources for        announcing of discovery information are allocated on a per UE        specific basis, further characterized by: i) The UE in        RRC_CONNECTED may request resource(s) for announcing of        discovery information from the eNB via RRC, ii) The eNB assigns        resource(s) via RRC, iii) The resources are allocated within the        resource pool that is configured in UEs for monitoring.

For UEs in RRC_IDLE, the eNB may select one of the following options:

-   -   The eNB may provide a Type 1 resource pool for discovery        information announcement in SIB. UEs that are authorized for        Prose Direct Discovery use these resources for announcing        discovery information in RRC_IDLE.    -   The eNB may indicate in SIB that it supports D2D but does not        provide resources for discovery information announcement. UEs        need to enter RRC Connected in order to request D2D resources        for discovery information announcement.

For UEs in RRC_CONNECTED,

-   -   A UE authorized to perform ProSe Direct Discovery announcement        indicates to the eNB that it wants to perform D2D discovery        announcement.    -   The eNB validates whether the UE is authorized for ProSe Direct        Discovery announcement using the UE context received from MME.    -   The eNB may configure the UE to use a Type 1 resource pool or        dedicated Type 2 resources for discovery information        announcement via dedicated RRC signaling (or no resource).    -   The resources allocated by the eNB are valid until a) the eNB        de-configures the resource(s) by RRC signaling or b) the UE        enters IDLE. (FFS whether resources may remain valid even in        IDLE).

Receiving UEs in RRC_IDLE and RRC_CONNECTED monitor both Type 1 and Type2 discovery resource pools as authorized. The eNB provides the resourcepool configuration used for discovery information monitoring in SIB. TheSIB may contain discovery resources used for announcing in neighborcells as well.

Recently, the extension of network coverage using L3-based UE-to-NetworkRelay is expected to be supported. When the UE starts ProSecommunication within the network and then moves out of the coverage, therelay may be selected by the UE or the network for service coverageextension. During changing the traffic path of the (potential) remote UEfrom eNB to a relay, there could be service interruption if the relayingservice activation (including relay selection) for the remote UE isperformed too late. On the contrary, if the relaying service activationis performed early, the remote UE might have dual connectivity for thesame (or different) PDN connection(s) where one connectivity goesthrough the eNB and another goes through relay. In addition, the(potential) UE may establish unnecessary connection between relay.

The serving cell of a relay and a remote UE might be different.Depending on whether the network supports the resource coordinationbetween the serving cell of the relay and the serving of the remote UE,it would be beneficial to limit the relay selection to the relaybelonging to the same cell of the cell of the remote UE.

FIG. 13 is an exemplary diagram for explaining the D2D environment.Referring to FIG. 13, discovery and communication between relay UE andthe remote UE is possible even in a scenario where two UEs are stayingin different cells. However, under the scenario of cell for ProSe of L2relay is different from cell for ProSe of the remote UE is different, itmay not be possible to perform direct discovery and direct communicationsince the L2 configuration and resource are configured by Cell 2 whileremote UE transmits the message with the synchronization of Cell 1.

To solve the problem, it is proposed of method of selecting thesynchronization reference for transmitting discovery/communicationmessage by the remote UE. This invention may be applied to the remote UEwhich are staying in (camping on/connected to) another/same cell fromthe cell of the connected relay UE. Specific details will be describedbelow.

In the present invention, the following may be assumed.

The sidelink procedure is operated in serving carrier of the remote UEor in dedicated carrier for sidelink operation. The technology forsidelink operation involves LTE sidelink and UE to network relaying overnon-3GPP access (e.g. using WiFi/Bluetooth).

-   -   The relay UE may be UE to UE relay as well as UE to network        relay.    -   The sidelink may refer to the link between relay UE and remote        UE.    -   The remote UE may be currently connected to the network via        relay UE.    -   The example measurement result of sidelink may be SD-RSRP/RSRQ,        RSSI, SINR and/or RSRP/Q. Other measurement result can be also        used. In case of relaying over non-3GPP RAT, the measurement        could be performed for the beacon or data part.    -   The remote UE may transmit to/receive from the same cell.

Network Assisted Autonomous Selection Method

FIG. 14 is a flow chart according to an embodiment of the presentinvention.

Referring to FIG. 14, the remote UE may receive a message for a sidelinkresource configuration (S1410). Specifically, since the remote UE in acoverage of cell 1 is connected to the cell 2 through the relay UE in acoverage of cell 2, the remote UE may receive the message from the cell2 via the relay UE. For this, it may be assumed that if remote UE islinked/connected to relay UE, the cell 2 is able to know existence ofthe connected remote UE and configure the remote UE (e.g. L1/2configuration, sidelink resource configuration). In this case, the cell2 which provides configuration used for sidelink communication/discoveryis regarded as serving cell to the remote UE. Serving cell of the remoteUE is same as that of the connected relay UE.

In addition, the remote UE may be provided with predetermined threshold(e.g. RSRP/RSRQ threshold) for determining which cell to be used forsynchronization reference cell for transmitting ProSe messages (e.g.discovery/communication message) via broadcast/dedicated signalling fromcell 2.

Subsequently, the remote UE may select a synchronization reference cell(S1420). Specifically, the remote UE may select a synchronizationreference cell using a predetermined threshold. For this, the remote UEmay measure a channel quality of the cell 2 or one or more neighboringcells of the remote UE.

For example, if channel quality of the cell 2 of the remote UE is lessthan the provided predetermined threshold (e.g. RSRP/RSRQ orRSRP/RSRQ−hysteresis value (if configured)), the remote UE may selectother cell as synchronization reference cell. As an example, the cell 1may be the synchronization reference cell. As another example, the othercell may be best ranked cell on the frequency of channel quality isbeing measured for the above comparison. Otherwise, if channel qualityof the cell 2 is higher than or equal to the predetermined threshold(e.g. RSRP/RSRQ or RSRP/RSRQ+hysteresis value (if configured)), theremote UE may select the cell 2 as synchronization reference cell.

After selecting the synchronization reference cell, the remote UE mayperform synchronization to the indicated cell when transmitting sidelinkcommunication, sidelink discovery or synchronization information.

Subsequently, the remote UE may transmit a sidelink data to the relayUE. Specifically, the remote UE may transmit the sidelink data to thesecond UE using the sidelink resource configuration received from thecell 2 based on the synchronization acquired from a synchronizationsignal of the cell 1.

According to an embodiment of the present invention, the interferenceproblem caused by transmitting, by the remote UE in the coverage of thecell 1, a signal based on the synchronization of cell 2 can be solved inthe coverage of the cell 1.

Network Based Selection Method

According to another aspect, the remote UE may receive the instructionof the network and select the synchronous reference signal based on thereceived instruction.

The remote UE may report information of the channel quality (e.g.RSRP/RSRQ measurement) to the network. Based on the reported informationof the remote UE, the network may determine the synchronizationreference cell for the remote UE and indicates the selectedsynchronization reference cell identity.

For example, the cell 2 may provide RSRP/RSRQ threshold for reportingthe measurement results of serving cell and/or neighbor cells to theremote UE. If the channel quality of the cell 2 is less than or equal tothe provided threshold (e.g. event A2), the remote UE may report thechannel quality of cell 2 and available channel quality of bestneighboring cell on the same frequency and the associated cell identityto the network. If the channel quality of the cell 2 is higher than theprovided threshold (e.g. event A1), the remote UE may report theavailable channel quality of cell 2 and/or neighbor cells and theassociated cell identity.

Subsequently, the network may indicate the cell identity of reportedbest ranked neighbor cell in the same frequency of the current cell tobe used for the synchronization reference cell of the remote UE. If thecell 1 is selected as synchronization reference cell of the remote UE,the network provides cell 1 as synchronization reference cell of theremote UE. If the cell 2 is selected as synchronization reference cellof the remote UE, the network might omit the signaling. If there is nosynchronization reference cell via dedicated signalling, the remote UEconsider the current cell as synchronization reference cell fortransmitting communication/discovery message.

Subsequently, the UE may perform synchronization to the indicated cellwhen transmitting sidelink communication, sidelink discovery orsynchronisation information.

FIG. 15 is a flowchart for explaining an embodiment of the presentinvention.

In this example, it is assumed that best ranked cell of the remote UE isCell1 and at first remote UE is connected to Cell1. Referring to FIG.15, the relay UE may be connected to/served by Cell2. Measurement forsidelink relay is triggered by comparing serving cell RSRP/RSRQ withRSRP/RSRQ threshold as in Rel-13 relay discovery in remote UE. It may bealso assumed that the serving cell measurement and intra-frequencyneighbor cell measurement is currently being performed.

When the remote UE finds the candidate relay UE(s), the remote UE mayinform the found candidate relay UE(s) to the currently connected cell(i.e. Cell1) with the assistance information (e.g. found relay UE ID,link quality between remote UE and found relay UE(s), cell identity ofrelay UE(s) and remote UE ID). Remote UE ID may refer to the existingProSe UE ID or new identity.

Subsequently, Cell2 may be provided with UE context information fromCell1 since Cell2 is required to make a decision on UE configuration asin legacy handover. After receiving UE context information, Cell2provides configuration of remote UE, sidelink configuration (e.g.resource configuration, sync configuration) to Cell1. The syncinformation includes RSRP/RSRQ threshold and optionally hysteresisvalue. Then, Cell1 forwards the received configuration to the remote UE.

After receiving the configuration from Cell2, the remote UE considersitself being connected to the Cell2. From the relay UE point of view, inorder not to filter out the message from remote UE, the network mayprovide the remote UE identity (L2 identity, e.g. ProSe UE ID). Theremote UE filter out other sidelink communication messages from theremote UE which the relay UE does not know.

If RSRP of Cell2 is lower than or equal to given threshold, the remoteUE selects best ranked intra-frequency neighbour cell (i.e. Cell1 inthis example) as synchronization reference cell. Otherwise, the remoteUE selects Cell2 as synchronization reference cell.

FIG. 16 is a block diagram of a communication apparatus according to anembodiment of the present invention.

The apparatus shown in FIG. 16 can be a user equipment (UE) and/or eNBadapted to perform the above mechanism, but it can be any apparatus forperforming the same operation.

As shown in FIG. 16, the apparatus may comprise a DSP/microprocessor(110) and RF module (transceiver; 135). The DSP/microprocessor (110) iselectrically connected with the transceiver (135) and controls it. Theapparatus may further include power management module (105), battery(155), display (115), keypad (120), SIM card (125), memory device (130),speaker (145) and input device (150), based on its implementation anddesigner's choice.

Specifically, FIG. 16 may represent a UE comprising a receiver (135)configured to receive a request message from a network, and atransmitter (135) configured to transmit the transmission or receptiontiming information to the network. These receiver and the transmittercan constitute the transceiver (135). The UE further comprises aprocessor (110) connected to the transceiver (135: receiver andtransmitter).

Also, FIG. 16 may represent a network apparatus comprising a transmitter(135) configured to transmit a request message to a UE and a receiver(135) configured to receive the transmission or reception timinginformation from the UE. These transmitter and receiver may constitutethe transceiver (135). The network further comprises a processor (110)connected to the transmitter and the receiver. The processor (110) isconfigured to perform operations according to the embodiment of thepresent invention exemplarily described with reference to theaccompanying drawings. In particular, the detailed operations of theprocessor (110) can refer to the contents described with reference toFIGS. 1 to 15.

The embodiments of the present invention described herein below arecombinations of elements and features of the present invention. Theelements or features may be considered selective unless otherwisementioned. Each element or feature may be practiced without beingcombined with other elements or features. Further, an embodiment of thepresent invention may be constructed by combining parts of the elementsand/or features. Operation orders described in embodiments of thepresent invention may be rearranged. Some constructions of any oneembodiment may be included in another embodiment and may be replacedwith corresponding constructions of another embodiment. It is obvious tothose skilled in the art that claims that are not explicitly cited ineach other in the appended claims may be presented in combination as anembodiment of the present invention or included as a new claim bysubsequent amendment after the application is filed.

In the embodiments of the present invention, a specific operationdescribed as performed by the BS may be performed by an upper node ofthe BS. Namely, it is apparent that, in a network comprised of aplurality of network nodes including a BS, various operations performedfor communication with an MS may be performed by the BS, or networknodes other than the BS. The term ‘eNB’ may be replaced with the term‘fixed station’, ‘Node B’, ‘Base Station (BS)’, ‘access point’, etc.

The above-described embodiments may be implemented by various means, forexample, by hardware, firmware, software, or a combination thereof.

In a hardware configuration, the method according to the embodiments ofthe present invention may be implemented by one or more ApplicationSpecific Integrated Circuits (ASICs), Digital Signal Processors (DSPs),Digital Signal Processing Devices (DSPDs), Programmable Logic Devices(PLDs), Field Programmable Gate Arrays (FPGAs), processors, controllers,microcontrollers, or microprocessors.

In a firmware or software configuration, the method according to theembodiments of the present invention may be implemented in the form ofmodules, procedures, functions, etc. performing the above-describedfunctions or operations. Software code may be stored in a memory unitand executed by a processor. The memory unit may be located at theinterior or exterior of the processor and may transmit and receive datato and from the processor via various known means.

Those skilled in the art will appreciate that the present invention maybe carried out in other specific ways than those set forth hereinwithout departing from essential characteristics of the presentinvention. The above embodiments are therefore to be construed in allaspects as illustrative and not restrictive. The scope of the inventionshould be determined by the appended claims, not by the abovedescription, and all changes coming within the meaning of the appendedclaims are intended to be embraced therein.

What is claimed is:
 1. A method for acquiring, by a first user equipment(UE) in a coverage of a first cell, synchronization in a wirelesscommunication system, the method comprising: receiving a message for asidelink resource configuration, transmitted from a second cell througha second UE, wherein the second UE is in a coverage of the second cell;when a channel quality of the second cell is less than a predeterminedthreshold, received from the second cell, selecting the first cell as asynchronization reference cell; and transmitting a sidelink data to thesecond UE using the sidelink resource configuration, wherein thesidelink resource configuration is based on the synchronization acquiredfrom a synchronization signal of the first cell.
 2. The method of claim1, wherein when the channel quality of the second cell is higher than orequal to the predetermined threshold, the first UE selects the secondcell as the synchronization reference cell.
 3. The method of claim 1,further comprising: measuring a channel quality of the second cell andone or more neighboring cells of the first UE.
 4. The method of claim 3,wherein when the channel quality of the second cell is less than thepredetermined threshold, the first UE selects one of the one or moreneighboring cells as the synchronization reference cell based on thechannel quality.
 5. The method of claim 1, further comprising: acquiringthe synchronization from the synchronization signal of the first cell.6. The method of claim 1, wherein the first UE is connected to thesecond cell through the second UE.
 7. A first user equipment (UE) in acoverage of a first cell in a wireless communication system, the firstUE comprising: a transceiver; and a processor, operatively connectedwith the transceiver, wherein the processor is configured to: controlthe transceiver to receive a message for a sidelink resourceconfiguration, transmitted from a second cell through a second UE,wherein the second UE is in a coverage of the second cell, when achannel quality of the second cell is less than a predeterminedthreshold, received from the second cell, select the first cell as asynchronization reference cell, and control the transceiver to transmita sidelink data to the second UE using the sidelink resourceconfiguration, wherein the sidelink resource configuration is based onthe synchronization acquired from a synchronization signal of the firstcell.
 8. The UE of claim 7, wherein when the channel quality of thesecond cell is higher than or equal to the predetermined threshold, thefirst UE selects the second cell as the synchronization reference cell.9. The UE of claim 7, wherein the processor is further configured to:measure a channel quality of the second cell and one or more neighboringcells of the first UE.
 10. The UE of claim 9, wherein when the channelquality of the second cell is less than the predetermined threshold, thefirst UE selects one of the one or more neighboring cells as thesynchronization reference cell based on the channel quality.
 11. The UEof claim 7, wherein the processor is further configured to: acquire thesynchronization from the synchronization signal of the first cell. 12.The UE of claim 7, wherein the first UE is connected to the second cellthrough the second UE.